Silicon solar cells with efficiencies higher than 15% are typically made of multicrystalline silicon or single crystalline silicon, which can be either grown by Czochralski or Float-Zone technique. When manufacturing costs of solar cells need to be minimized, it is used the multicrystalline silicon. Solar cells made from Czochralski and float-zone silicon are comparable in cost, but Float-Zone silicon is used for high-efficiency applications.
It has been studied that both Czochralski and Float-Zone solar cells suffer from an unstable efficiency, which tends to degrade under sunlight when a minority-carrier lifetime in the solar cells reduces. This phenomenon is known as a light induced minority-carrier lifetime degradation and it is currently a serious problem limiting a solar cell efficiency.
It is known that the light induced minority-carrier lifetime degradation has a clear dependency on a copper concentration regardless of the silicon material. The lifetime degradation due to copper can be explained by the fact that the copper has a high diffusivity in silicon also at room temperature. Light activation of the interstitial copper reduces an electrostatic repulsion between positively charged interstitial copper ions and copper precipitates, which enables copper to precipitate in the wafer bulk even at a low concentration level. Such formation of the copper precipitates increases the recombination activity, which naturally has a strong negative effect on the solar cell efficiency.